The Brazil forward had just received the ball on the left when he raised his hand to seemingly signal to his coach Thomas Tuchel he was in pain, and looked to be in tears as he left the field at Parc des Princes.
A few hours after Neymar was taken to hospital, PSG said in a statement in the early hours of Thursday that "the first tests carried out showed a painful reactivation" of the fifth metatarsal in the right foot he fractured nearly exactly one year ago playing against Marseille.
PSG added that Neymar's condition will be further assessed "in the coming days" to determine how long he will be out of the lineup. PSG travels to face a resurgent United side at Old Trafford on Feb. 12 with the return leg in Paris on March 6.
"All therapeutic options need to be envisaged," PSG said on its website.
In February of last year, Neymar cracked the fifth metatarsal in his right foot, and he underwent surgery back home in Brazil. Although he recovered in time for the World Cup, he was not at his best in Russia.
Neymar limped heavily as he walked to the dressing room on Wednesday night, consoled by a PSG staff member, and looked visibly upset as he wiped his face with his right arm following his third significant injury in the space of a calendar year.
It was unclear how Neymar sustained his latest injury but he had been on the receiving end of a heavy tackle from midfielder Anthony Goncalves earlier in the match. Television images then later showed Neymar rolling off his sock and pointing to the side of his right foot.
A frustrated Tuchel said Neymar was not protected enough on the pitch.
"The referee didn't whistle on one occasion, then two, then three and after that he ended up twisting his foot," Tuchel said.
But interviewed after the match by broadcaster Eurosport, Goncalves expressed no sympathy for Neymar and even suggested the injury is a direct result of how he plays.
"It's his style. But if you play like that don't complain if you take some knocks after," Goncalves said. "He's a great player, and I respect the player he is, but we're not here so he has fun at our expense. We're not here to make him look good."
Goncalves suggested Neymar, who has a tendency to showboat and even toy with opposing players with his superior level of skill, lacks humility.
"If he wants to have fun, we'll respond with the weapons we have. We have a jersey to defend. We're not here for a laugh," Goncalves said. "He can amuse himself, that's his style and I think people come to watch that. But he shouldn't come and moan about it afterward...."
Strasbourg coach Thierry Laurey also said Neymar's attitude is all wrong.
"Sometimes when you've gone too far, you have to face up to it," Laurey said. "I can understand my players having enough of players who are looking to tease them, mock them. It's right that after a while it also catches up with him. I'm all for protecting players, but there are limits. When you make a pass that comes off your back, you're mocking a bit."
Strasbourg defender Pablo Martinez added: "Of course he's a marvelous player, but because of his attitude he's not someone I like on the pitch. Apart from teasing you, he talks to you badly."
PSG is already sweating on the fitness of key midfielder Marco Verratti after he sprained his ankle last weekend.
Two months ago, Neymar was substituted with a groin injury during the first half of Brazil's friendly against Cameroon on Nov. 20.